A solid material is known that emits light when it is radiated with an excitation beam. For example, a solid material doped with rare earth element such as Nd:YAG, Yb:YAG, Tm:YAG, Nd:YVO4, Yb:YVO4, Nd:(s-)FAP, Yb:(s-)FAP, Nd:glass, and Yb:glass, or a solid material doped with transition element such as Cr:YAG and Ti:Al2O3 emit strong light when radiated with the excitation beam. These types of the solid materials may be arranged in a resonator that resonates at a particular wavelength to obtain a laser resonator.
A solid material is also known that emits an output laser beam when it is radiated with an excitation beam and an input laser beam, the output laser beam being amplified from the input laser beam. When this type of the solid material is used, a laser amplifier is thereby obtained. In the description herein, these two types of the solid materials are termed optical gain materials.
Further, a solid material is known that emits an output laser beam with a different wavelength from that of an input laser beam when it is radiated with the input laser beam. When this type of the solid material is used, a wavelength converter is thereby obtained. In the description herein, this type of the solid material is termed a nonlinear optical material.
In the description herein, the optical gain material and the nonlinear optical material will collectively be termed a laser medium.
The laser medium under operation generates heat. Especially the optical gain material generates a large amount of heat due to its quantum defects accompanying excitation. When the laser medium is overheated, resonating efficiency is deteriorated due to uneven distribution of refractive indexes within the laser medium, thermal lens effect caused by thermal expansion, and also issues related to thermal birefringence caused by photoelastic effect, and the laser medium is damaged in the end due to stress thereon. Because of reasons as above, cooling is essential in a solid laser device or the like that uses the solid material. Further, in order to prevent beam quality of the laser beam from being deteriorated, the laser medium not simply needs to be cooled but also preventions are necessary for generation of warping and the like inside the laser medium, and to achieve this measure, the cooling needs to be performed so that a temperature distribution inside the laser medium is uniformized. A cooling technique is essential to facilitate high laser beam output, and a technique configured to cool the laser medium effectively and with a uniform temperature distribution is required.
U.S. Pat. No. 5,796,766 describes a laser component provided with a function to cool a laser medium. In this technique, the laser medium has a circular disc shape, and it transmits heat to a transparent heat transmitting member similarly given a circular disc shape. In the description herein, one flat surface of the circular disc-shaped laser medium will be termed a first end surface, and another flat surface thereof will be termed a second end surface. In the technique of U.S. Pat. No. 5,796,766, a circular disc-shaped first heat transmitting member is brought into contact with the first end surface of the circular disc-shaped laser medium, a circular disc-shaped second heat transmitting member is brought into contact with the second end surface of the circular disc-shaped laser medium, and the laser medium is cooled from both the first and second end surfaces.
U.S. Pat. No. 5,796,766 describes methods for making the laser medium and the heat transmitting member contact each other, including: (1) a method of making both members contact each other by mechanical force (which U.S. Pat. No. 5,796,766 describes as “optical contact”), (2) a method of adhering both members by adhesive, (3) a method of fixing both members by epoxy resin, and (4) a method of diffusion bonding both members.
It has been found from studies conducted by the present inventors that aforementioned methods (1) to (3) cannot sufficiently cool the laser medium due to high thermal resistance between the laser medium and the transparent heat transmitting member. That is, it has been found that the laser beam intensity cannot be increased to a level needed by users of the laser device due to overheating of the laser medium. In case of (1), due to discontinuity of substances at an interface, phonons are dispersed by this discontinuous interface. That is, an increase in the thermal resistance thereby occurs, and this makes it unable to provide essential solution. Further, the adhesive and the epoxy resin layer in (2) and (3) create thermal resistance. Further, they exhibit serious damage issue due to deterioration of the resin upon high-power operation. According to the method of (4), although the thermal resistance between the laser medium and the heat transmitting member can sufficiently be reduced, because they are bonded under a high temperature, a difference in thermal expansion coefficients of the laser medium and the heat transmitting member causes strong residual stress to act on the laser medium under room-temperature operation. The residual stress causes optical distortion in the laser medium which deteriorates beam quality.
In view of the above, a technique of surface-active bonding the laser medium and the transparent heat transmitting member has been developed, and such is described in Hiroki TOGASHI, “Creation and Evaluation of Yb:YAG/Diamond Composite Structured Laser Using Normal Temperature Bonding”, Master's Thesis, Chuo University (2013) (hereafter “TOGASHI”). In TOGASHI, YAG which is one type of laser medium and diamond which is one type of transparent heat transmitting member are surface-active bonded. The surface-active bonding may be termed room temperature bonding or normal temperature bonding since the members are brought into contact without heating.
In this description, “surface-active bonding” refers to radiating inert gas atomic beam to bonding surfaces of both members to be bonded to thereby activate the bonding surfaces, bringing the activated bonding surfaces into contact with one another, and causing the two members to bond at their atomic levels by atomic bonds that appeared on the activated bonding surfaces. According to this surface-active bonding method, bonding can be performed under normal temperature, and the issues related to residual stress will not arise. Further, the bonding taking place at atomic levels can sufficiently reduce thermal resistance between the two members. Other references, Eiji HIGURASHI, Ken OKUMURA, Kaori NAKASUJI, and Tadatomo SUGA, “Surface activated bonding of GaS and SiC wafers at room temperature for improved heat dissipation in high power semiconductor lasers”, Japanese Journal of Applied Physics, 54 030207 (2015) (hereafter “HIGURASHI et. al”) and Yoichi SATO, Akio IKESUE and Takunori TAIRA, “Tailored Spectral Designing of Layer-by Layer Type Composite Nd:Y3ScAl4O12/Nd: Y3Al5O12 Ceramics”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 13, No. 3 May/June (2007) (hereafter “SATO et. al”), will be described later.